The present disclosure relates to systems and methods for magnetic resonance imaging (“MRI”). More particularly, disclosure relates to systems and methods for designing and implementing data acquisition schemes for MRI that sample k-space along a series of azimuthal equidistant projections.
Radial MRI permits the use of higher undersampling factors and is more tolerant to physiologic motion artifacts than Cartesian MRI. The traditional method of sampling radial k-space involves acquiring projections that are evenly spaced in the azimuthal direction. For instance, radial acquisitions can include acquiring a projection every 180/N degrees, where N is the number of projections in the acquisition.
Although the conventional radial MRI approach provides satisfactory image quality in many circumstances, it can lead to artifacts from physiologic motion, including arterial pulsation, respiration, or other spurious motion. Artifacts can also arise from evolving spin magnetization because the projections that are degraded by motion are closely clustered together in k-space and materially degrade image quality.
Methods to randomly or pseudorandomly sample radial k-space and suppress these image artifacts result in artifacts related to eddy currents. To minimize such artifacts, the radial trajectory utilized should distribute projections around the azimuthal space in a substantially uniform manner, while simultaneously providing for evenly spaced projections using a constant, relatively small azimuthal distance between the acquisition of successive projections.
An optimal solution to this radial projection ordering problem is not easily obtainable and requires the use of significant computational effort that is time consuming and ill-suited for applications in the clinical setting. There thus remains a need to provide a method for efficiently computing the azimuthal angular increments that provide for azimuthal substantially equidistant projections.
Other methods for providing substantially uniform radial k-space sampling while distributing successive projections over the radial k-space domain have been proposed. One such method involves the use of a golden angle azimuthal projection increment of around 111.25 degrees. Although the use of a golden azimuthal angle increment generally provides widespread sampling of k-space, the method does not provide substantially equidistant projections in radial k-space and leaves gaps of k-space un-sampled, which results in image artifacts. Furthermore, the use of a large golden azimuthal angle increment of around 111.25 degrees also requires large changes in the imaging gradients from projection to projection, which results in eddy currents that degrade image quality.
Another method involves the use of a trajectory that “pseudorandomly” samples radial k-space. This approach, however, does not ensure the acquisition of azimuthal substantially equidistant projections. In particular, this method requires the user to judiciously pre-select five parameters for use in a lengthy analytical expression. At present, there is no known method for properly selecting these five parameters for a given radial scan. Improper parameter selection results in the acquisition of overlapping (i.e., duplicated) projections and the exacerbation of artifacts from radial undersampling.
It would therefore be desirable to provide systems and methods that are capable of designing and implementing radial k-space acquisitions using azimuthal substantially equidistant projections.